1. Field of the Invention
The present invention relates generally to an antenna array with adaptive sidelobe cancellation and more particularly to a beam space antenna array with adaptive subarraying.
2. Description of the Prior Art
The need for early and accurate detection of hostile targets has created a need for radar antennas that will function in the presence of incoming signals from unwanted interference sources (jammers). Jamming signals received in the sidelobes of the antenna obsure target echos in the mainlobe. Thus, jammer signals received in the antenna sidelobes must be cancelled or nulled in order to effectively detect targets.
A fully adaptive array may be utilized to achieve sidelobe cancellation. An adaptive array is an antenna that senses the received signal illuminating its aperture and adjusts the phase and amplitude of the aperture illumination to achieve some desired performance criterion. In a fully adaptive array each antenna element has a separate adaptive weight setting. A fully adaptive sidelobe canceller has the theoretical capability of nulling a large region in spaced. However, the large number of degrees of freedom in a fully adaptive array increase the complexity and convergence time of the adaptive network.
Further, fully adaptive antenna arrays may be configured as beam space systems wherein each of the output ports responds to a signal source in a predetermined angular direction relative to the array. This is accomplished by utilizing a phase shifting device, preferably a Weighted Butler Matrix, that has a plurality of input ports and output ports where each of the input ports is coupled to a respective antenna element. The phase shifting device generates an orthogonal set of subbeams, each responding to a particular output port of the Weighted Butler Matrix. One of the subbeams is regarded as the main beam (or main channel) while the other subbeams are adaptively weighted by a multiple sidelobe canceller to form a cancellation beam which is subtracted from the main beam. This cancellation beam cancels signals incident on the sidelobes of the main channel. Each of the other subbeams that feed the multiple sidelobe canceller has a null in the direction of look of the main beam; and therefore, the output has a constant response to the direction of look of the main beam.
The beam space fully adaptive array is particularly advantageous in that it discriminates effectively against interfering noise sources in the near-in sidelobe regions, which includes the sidelobes near the main beam of the antenna pattern, as well as in the far sidelobe region which includes those sidelobes farthest from the main beam of the antenna.
However, such fully adaptive beamspace arrays require extensive hardware in their implementation, in that an adaptive circuit is required for each of the individual antenna elements. Additionally, for an antenna with a large number of elements, N, the adaptive processor must perform the equivalent of solving a set of N simultaneous linear equations. The convergence rate of the solution to these equations becomes prohibitively slow as N increases.
In order to reduce the degrees of freedom, i.e., input signals to the sidelobe canceller, and to increase the convergence rate, a partially adaptive beam space system may be utilized.
In a partially adaptive array either a small number of the total antenna elements are provided with adaptive circuitry or the elements are controlled in groups (subarraying). If only a small number of elements are controlled these elements are designated auxiliary elements.
However, if only auxiliary elements are chosen as inputs to the sidelobe canceller, jammer nulling is effected by the bandwidth-aperture product, i.e., if the number of antenna elements is held constant, as the bandwidth of the jammer or the size of the antenna aperture in terms of wavelengths or the spacing of the auxiliary elements is made larger, the nulling of the jammer is degraded. This degradation results because as auxiliary antenna elements are spaced farther apart the input noise signal with a spread spectrum becomes more uncorrelated between elements. However, it is the correlation between the signals on the respective antenna elements that allows the adaptive array to sense the direction of arrival of an undersirable signal and then place a null in the main channel sidelobe oriented in the direction of that undesirable signal.
Thus, these partially adaptive antenna systems are capable of discriminating against at most only that number of undesirable signal sources that correspond to the number of selected antenna elements, and will not fully be effective in discriminating against more than this number within the field of view of the antenna array. Additionally, these systems increase the sidelobe level of the main channel due to the spacial ambiguity of the undesired signal with respect to the auxilary antenna elements.